1. Field of the Invention
The invention relates to a force sensor for measuring acceleration or force applied to a body or the pressure of a fluid, a temperature sensor for measuring the temperature of various atmospheres or the like, and a temperature/force sensor device provided with these functions. More particularly, the invention relates to a sensing mechanism which uses the cold emission.
2. Discussion of the Related Art
Sensors for measuring a physical quantity in various atmospheres are requested to have an improved sensitivity, the compatibility with the atmosphere under measuring, etc. Under this situation, new type sensors have been developed. A sensor shown in FIGS. 34 and 35, for example, is a device for detecting a pressure applied to the device using the piezoresistance effect of a resistor (conductor or semiconductor) which is a phenomenon wherein the electric resistance of the resistor is changed depending on the change of the resistivity due to the pressure. Such a device is mainly employed in measuring acceleration applied to a body, and therefore called an acceleration sensor. The acceleration sensor is a semiconductor sensor in which the resistor is made of a semiconductor. Specifically, an Si (silicon) substrate 141 has a thin portion at its center, and an air gap 148 is formed so as to surround the thin portion except a supporting unit 151. A weight 152 is fixed to the back of the thin portion and functions as a moving unit 142.
On the surface of the supporting unit 151, disposed are piezoresistance units 143 which detect their deformation (the change in resistivity). The piezoresistance units 143 are connected to a lead wire 149 through a diffusion conductor layer 144 and bonding pads 145. The Si substrate 141 is attached by the technique of electrostatic adhesion or the like to a glass cover 146 having a cavity 147, thereby constructing the semiconductor acceleration sensor. When a pressure due to acceleration of a body is applied to the thus configured semiconductor acceleration sensor, the supporting unit 151 elastically deforms to bend, and the moving unit 142 is displaced in accordance with the bend. This causes the electric resistances of the piezoresistance units 143 disposed on the surface of the supporting unit 151 to change in proportion to the deform of the supporting unit 151. Accordingly, the acceleration can be measured from the change of the electric resistances.
FIG. 36 shows another prior art pressure sensor, and FIG. 37 shows in an enlarged scale a strain gauge unit of the sensor. The pressure sensor is a semiconductor pressure sensor in which the electric resistor is made of a semiconductor and which measures a pressure change of a fluid. In the semiconductor pressure sensor, a diaphragm unit 176 is formed by partially thinning an Si substrate 161. In the upper portion of the Si substrate, formed are a strain gauge unit (piezoresistance unit) 171, an operational amplifier unit 173 for amplifying a resistivity change of the strain gauge unit 171, and a thin-film resistor unit 174. The reference numeral 172 designates aluminum wiring patterns, and 175 designates a surface protection film. The Si substrate 161 configured as described above is placed on a base 167 through glass pedestals 164 having a thermal expansion coefficient which is similar to that of silicon, and then sealed in a metal case 166 while being connected to terminals 169 through Au wires 165. In the thus configured semiconductor pressure sensor, when the pressure of a fluid flowing through an introduction pipe 170 increases, the stress causes the diaphragm unit 176 to elastically deform so that the resistance of the strain gauge unit 171 disposed above the diaphragm unit 176 is changed in proportion to the stress (distortion). This resistivity change is amplified by the operational amplifier unit 173 to be output. The fluid pressure can be measured by converting the obtained resistivity change to the value of the stress.
Both the pressure sensors shown in FIGS. 34 and 37 are semiconductor pressure sensors in each of which the electric resistor is made of a semiconductor. The gauge factor of such a semiconductor pressure sensor which is an index to the sensitivity is several ten times that of a metal strain gauge pressure sensor having an electric resistor made of a metal wire or foil. Therefore, a semiconductor pressure sensor has advantages such as that the sensitivity is excellent, and that the output level is so high as to be easily amplified.
However, a prior art semiconductor pressure sensor uses the piezoresistance effect of an electric resistor. Even in a semiconductor pressure sensor having an Si substrate which is relatively excellent in sensitivity, therefore, the resistivity change is as small as about 2% when the rated output is applied, thereby producing a problem in that such a sensor cannot accurately measure a pressure. Moreover, since such a resistor shows a large change in physical property (large negative temperature characteristic) against temperature or pressure, a prior art semiconductor pressure sensor requires a temperature compensating circuit, thereby producing another problem in that such a sensor is inferior in environmental resistance. In a prior art pressure sensor, for example, under an environment of a high temperature (about 120.degree. C.), a high pressure (about 20 atm.), and radioactive rays, a large leak current flows irrespective of the degree of distortion due to a pressure so that such a prior art sensor cannot perform a highly reliable measurement under such conditions. There is a further problem in that the resistance of the piezoresistance unit easily fluctuates (about 1%/1,000 Hr).
In the same manner as a pressure sensor, also a temperature sensor is requested to have a higher environmental resistance to a high temperature, radioactive rays, or the like. Under the present status of the art, however, such a requirement has not yet been satisfied.